Projection- apparatus



April 10, 1928.

L. V. FOSTER PROJECTION APPARATUS Filed March 28. 1925 Patented Apr. 10, 1928.

UNITED STATES PATENT OFF-ICE LEON V. FOSTEE, OF ROCHESTER, NEW YORK, ASSIGNOR T BAUSCH & LOMB OPTICAL COMPANY, OF ROCHESTER, NEW YORK, A CORPORATION or new YORK.

PROJECTION APPARATUS.

Application filed 1mm 28, 1925. Serial 110'. 19,165.

The present invention relates to projection apparatus, and has for its ob ect to provide improvements in apparatus of this kind, particularly with respect to the reflecting means used in the illumination of motion pictures in difierent types of prqectmg machines.

Another object of the invention is to provide in combination with motion p1cture projection ap aratus an improved reflector for focusing t e' light to form a real image at the back of the projection lens so that the film gate will intercept the-beam of light where the even intensity of illumination is greatest over the entire aperture.

A further object of the invention is to provide, for use in motion picture projectlon apparatus, a reflecto'r embodyinga focusing mirror corrected for spherical aberration and therefore differing from the spherical mirror which is spherically undercorrected, the improved'res ts being accom hshed by combining with the reflectingsu ace of the mirror a negative lens spherically overcorrected, whereby the two aberrations neutralize one another, dueto the pro er coordma tion ofthe radii of the front an rear spherical curves of the mirror in con'unction with the use of a medium of high re active index between said surfaces. This combination affords a relatively inexpensive mirror which can be easily ground and polished with oralnaryopticians tools and which permits the light source to be used at a safe distance from the lass without sacrificing any of light or pro ucing uneven screen il umination.

To these and other ends the invention resides in certain improvements and combinations of parts, all as will be hereinafter more fully described, the novel features being pointed out in the claims at-the end of the specification.

In the drawings:

Figure 1 represents diagrammatically one embodiment of the invention, and

Figure 2 is an enlarged sectional view of the improved reflector.

Sim1lar reference numerals throughout the several views indicate the same parts.

In motion picture projection apparatus,

particularly in small theaters, the direct current reflecting arc lamp has been used for illuminating purposes due to the low consum tion of current with lampsofthis ty e Whig the reflector employed has gener y uneven screen illumination. 1

While a spherical reflectormay be free from spherical aberration if the object and image are at the center of curvature of the mirror, still such a reflector, under the same arrangement as above mentioned, roduces a condition of spherical undercorrection and suffers in the same manner as the parabolic reflector, though to a greater extent. The parabolic mirror" can be made lar e' enough to include a solid angle of light 0 120 but the spherical mirrorcannot be made to include more than 100 of useful illumination. The'surface of rotation in whichv an object is imaged by reflection, without spherical aber ration, would be ideal in arc lamp projection apparatus for motion picture projection.

Such a surface is an ellipsoid of revolution but is very difficult and expensive to make from glass. V V In the present invention I have illustrated diagrammatically a portion of a motion picture projection apparatus, includin a suitable projection lens 10 of'any we known type, a diaphragm llof any preferred form and my improved reflector, indicated generally at 12, along with a suitable source of light embodying the electrodes 13 and 14, wlth which may be employed a suitable feeding mechanism, not shown, the electrode 13 being projected through an opening 15 in the center of the mirror as shown in Fi re 1. The reflector 12 is one in which erical curves are used but with the radius 5i curvature of the rear surface 16, having a coatin 17, differing from that of the curvature o thefront surface 18. The present mirror,

by way of example is one in which the front parabolic mirror or spherical I I bon at a point on the axis thereof and the focal lengt of the mirror being 71.0 mm.

While this mirror is similar in shape to the well known Mangin mirror it differs therefrom in that the latter is adapted to project parallel rays of light while the present reector is a focusing mirror capable of projecting a concentrated light on theaxis of the projection lens to roduce a real image in or adjacent the bee of the lens so that the film gate will intercept the beam of light where the even intensity of. illumination is greatest over the entire aperture, as stated above.

The present mirror though analcgous to a spherical mirror, which is spherically undercorrec-ted, differs therefrom in having a combined negative lens which is spherically overcorrected, whereby the two aberrations are such as to neutralize. one another. This results in a mirror corrected for spherical aberration which is comparable with the corrections of the elli soid of revolution and affords a mirror whic 1 can be both accurate 1 and easily ground and polished and therefiire economically produced. This type of mirror can be made large enough to includev a solid angle of light of 120 which is desirable in motion picture projection apparatus.

An important feature of the present reflector is that it ermits the are or light source to be place" at a relatively safe dis tance from the mirror. As will be understood, there exists between a spherical and a parabolic mirror of the same focal length and relative aperture, a difference in shape, the parabolic mirror being more shallow than the Spherical mirror. This difference in shape permits the spherical mirror to be made smaller in diameter to give the same iative aperture as the parabolic mirror but the periphery of the former approaches nearer to the are. It is not practical to place a glass surface too close to the arc lamp, for reasons which are evident, nameiy, the heat of the arc may crack the glass and the cararticles thrown off from the arc may hit t e surface of the glass. The surface of the mirror as stated above is preferably three inches away from the arc crater, a suficient distance to avoid itting. In order to avoid breaking of the g ass, due to a sudden change of temperature, this mirror is preferably made from a material having a' low coeflicient of expansion, such as quartz or a heat resisting glass.

It is to be understood that other mirrors having the same correction for spherical aberration, hut of different focal lengths from the one mentioned above, may be made by applying a constant ratio to the curves, thickness and diameter. Such mirrors would have, a constant magnification, but the object and image distances wiil be in the same ratio as the focal length ratio. Furthermore, other mirrors having different corrections for spherical aberration where the ratio of the object to image distance, or magnification is different can also be constructed in keeping with the principles set forth herein. I

I claim as my invention:

1.- In a projection apparatus the combina tion with a projection lens and a light source, of a mirror adapted to form near the projection lens an image of the source of light, said mirror being formed with front and rear spherical surfaces separated by a medium of high refractive index, the front and rear surfaces having spherical undercorrection which is compensated for by overcorrection afforded by the medium of high refractiveindex lying between said surfaces.

2. A reflector adapted to form at a finite distance a real image of a source of light,

said reflector embodying front and rear surfaces having spherical undercorrection compensated for by overcorrection afforded by a medium of hi h refractive index lying between said sur aces, whereby to produce a mirror substantially free from spherical aberration for the said finite image distance.

3. A reflector for use in motion picture projection apparatus embodying front and rear spherical surfaces separated by a transparent medium of high refractive index, the rear surface being coated to form a mirror, the radii of curvature of said front and rear surfaces being substantially in the proportions of 76 to 113, res ectively.

L ON V. FOSTER. 

